International Scholarly Research Notices

Volume 2015 (2015), Article ID 484768, 11 pages

http://dx.doi.org/10.1155/2015/484768

## IMPATT Diodes Based on , , and Oriented GaAs: A Comparative Study to Search the Best Orientation for Millimeter-Wave Atmospheric Windows

^{1}Supreme Knowledge Foundation Group of Institutions, Mankundu, Hooghly, West Bengal 712139, India^{2}Institute of Radio Physics and Electronics, University of Calcutta, 92 APC Road, Kolkata 700009, India

Received 28 June 2014; Revised 15 January 2015; Accepted 19 February 2015

Academic Editor: Georgios Veronis

Copyright © 2015 Bhadrani Banerjee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The authors have carried out the large-signal (L-S) simulation of double-drift region (DDR) impact avalanche transit time (IMPATT) diodes based on , , and oriented GaAs. A nonsinusoidal voltage excited (NSVE) L-S simulation technique is used to investigate both the static and L-S performance of the above-mentioned devices designed to operate at millimeter-wave (mm-wave) atmospheric window frequencies, such as 35, 94, 140, and 220 GHz. Results show that oriented GaAs diodes are capable of delivering maximum RF power with highest DC to RF conversion efficiency up to 94 GHz; however, the L-S performance of oriented GaAs diodes exceeds their other counterparts while the frequency of operation increases above 94 GHz. The results presented in this paper will be helpful for the future experimentalists to choose the GaAs substrate of appropriate orientation to fabricate DDR GaAs IMPATT diodes at mm-wave frequencies.

#### 1. Introduction

Impact avalanche transit time (IMPATT) diodes are well recognized two terminal solid-state devices to deliver sufficiently high power at both microwave and mm-wave frequency bands [1]. Silicon is the most popular base material for IMPATT diodes from the point of view of its advanced process technology [2–6]. However, GaAs is another vital base semiconductor for IMPATT diodes at the both microwave and mm-wave frequencies. Since early seventies, several researchers have fabricated IMPATT diodes based on GaAs and obtained higher DC to RF conversion efficiency and better avalanche noise performance of those as compared to their conventional Si counterparts [7–11].

The carrier ionization rates in a semiconductor material are key parameters which govern the RF performance of IMPATT sources. The inequality in ionization rates of electrons and holes (i.e., ) in GaAs was first reported in late seventies [12]. Pearsall et al. [13] experimentally measured the carrier ionization rates in GaAs under the electric field along the normal to , , and oriented crystal substances. They reported different values of and for different orientations. Thus, it is evident from the above-mentioned report [13] that the carrier ionization rates in GaAs depend significantly on the crystal orientation of the substrate. Since the RF performance of IMPATT diode is strongly dependent on the carrier ionization rates of the base material, the same must be significantly influenced by the crystal orientation of the substrate (here GaAs). This fact encouraged the authors to carry out a comparative study on the L-S performance of DDR IMPATT diodes based on , , and oriented GaAs. Earlier in 1993, Pati et al. [14] investigated the high frequency properties of , , and oriented --, -- (single-drift region (SDR)), and --- (DDR) GaAs IMPATT diodes at both 35 and 60 GHz frequencies by using a small-signal (S-S) simulation technique based on drift-diffusion model. Though the S-S simulation provides substantial insight into the IMPATT operation, it has some intrinsic restrictions due to a number of unrealistic assumptions. Several important properties of IMPATT source admittance characteristics, RF power output, DC to RF conversion efficiency, and so forth, cannot be precisely determined from the S-S simulation. Thus L-S simulation is essential to acquire the above-mentioned properties accurately. Therefore in the present paper authors have used a nonsinusoidal voltage excited (NSVE) L-S simulation method developed by them [15–20] to investigate both the static (DC) and L-S characteristics of DDR IMPATTs based on , , and oriented GaAs at different mm-wave atmospheric window frequencies, such as 35, 94, 140, and 220 GHz.

#### 2. Large-Signal Modeling and Simulation Technique

Schematic of the one-dimensional (1-D) model of DDR IMPATT structure is shown in Figure 1. This 1-D model is used for the L-S simulation of the device. It is well known that the physical phenomena take place in the semiconductor bulk along the symmetry axis of the IMPATT devices. Thus the 1-D modeling and simulation of the device are absolutely justified. The fundamental time and space dependent device equations, that is, Poisson’s equation, current continuity equations, and current density equations, are simultaneously solved under L-S condition subject to appropriate time varying boundary conditions by using a double-iterative simulation method [15–20] based on 1-D finite difference method (FDM). The fundamental device equations are given bywhere and are the partial derivatives with respect to and , respectively ( and ); all other symbols are carrying their usual significance. A list of symbols is given in appendix at the end of this paper where the usual meaning of each symbol is provided. The avalanche generation rates of both types of charge carriers at the space point and at the time instant are given by